Element glint simulator

ABSTRACT

A glint simulator is disclosed for receiving and reflecting impinging radar energy to simulate the results of a real target. This allows ready monitoring of a radar response to angle and amplitude scintillation error. A single point source of angle scintillation or glint from the simulator may comprise a microwave antenna, a modulator, a noise modulation source and a modulation phase adjustment. Several simulator point sources may be utilized with a single noise modulator to provide a variable multiple-point target. Simulator antennas receive the radar energy, couple it through respective modulators wherein noise modulation may be introduced to reduce or enhance the energy, and reflect it back to the radar.

llited States Patent [1 1 Qash et al. 1 Sept. 18', 1973 [54] N-ELEMENTGLINT SIMULATOR 3,699,575 10/1972 Peters, Jr. et al 343/18 D [75]lnventors: Carlton H. Cash, Hazel Green;

James L Jemigan Huntsvme, both Przmary Exammer-Malcolm F. Hubler ofAttorney-Harry M. Saragovitz et a1.

[73] Assignee: The United States of America as represented by theSecretary of the [57] ABSTRACT Army, Washington, DC A glint Simulator isdisclosed for receiving and reflect- [22] Filed: Apr. 17 1972 ingimpinging radar energy to simulate the results of a real target. Thisallows ready monitoring ofa radar rel l PP NOJ 244,687 sponse to angleand amplitude scintillation error. A single point source of anglescintillation or glint from the 52 us. Cl. 343/18 E 35/104 343/171simulaw' may a microwave antenna 343/l8 lator, anoise modulation sourceand a modulation [51] Hnt. Cl. G015 9/02 phase adjustment SeveralSimulator point sources may of Search 343/18 E 18 D 7 be utilized With3. single 110186 modulator to provide a 5 variable multiple-pointtarget. Simulator antennas receive the radar energy, couple it throughrespective [56] References Cited modulators wherein noise modulation maybe introduced to reduce or enhance the energy, and reflect it UNITEDSTATES PATENTS back to the fade 3,568,194 3/1971 Wilson et al. 343/18 E3,158,862 11/1964 Chisholm 343/18 D 3 Claims, 11 Drawing Figure [6 l8 mf T f I20 I4 22 d E E 12C l4 l6 ANTENNA MODULATOR SLlDlNG SHORT IZA 14he IB N-ELEMENT GLINT SIMULATOR BACKGROUND OFTHE INVENTION In radartracking of a target several measurement errors are encountered'fromcharacteristics of the target itself. Angular scintillation, glint, isthe wandering of the center of. reflection of a target from itseffective center. This appears on a radar display as an apparentdisplacement of the target from its mean position. The displacement mayconsistently vary as is well known in the art. For targets of severalwavelengths long, such as aircraft, the apparent radar mean position,center, will wander as the target rotates relative to the line-of-sight.When a target is in a radar shorbrairge region, glint becomes a dominantproblem for targets extending over several wavelengths. Radar echosreceived from a single target can be from several points on the target,indicating a rotating group of isolated points and disclosing anappparent multiple-point target. The reflecting points of amultiple-point target are fixed with respect to the target mean positionbut appear to wander relative to the radar line-of-sight. Whenuncompensated for, glint can cause a moving target to appear outside ofthe physical boundary, or extent, of the target.

SUMMARY OF THE INVENTION cause shifting of the effective reflectivepoints (simula-' tor antenna) on the target, resulting in apparentdisplacement of the target center from its mean position.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a diagrammaticview of a multiple-center target simulator.

DESCRIPTION OF THE PREFERRED EMBODIMENT An N-element simulator maycomprise from as few as two point sources to several point sources,depending on the surface of the actual target being simulated and theaccuracy of simulation desired. The single FIG- URE discloses apreferred embodiment of the invention wherein an N-element simulator isdisclosed having point sources 12A, 12B, 12C, and 12D. The sources 12are arranged in a common plane for reflecting energy substantially alonga planar path. Each point source 12 comprises an antenna 14 forreceiving and re-radiating radar energy, a modulator 16 for modulatingreceived radar energy, and a sliding short 18 for fine phase adjustmentof the modulator. Radar energy impinging on antennas 12 is coupled intomodulator 16, wherein the signal amplitude and phase is varied. Thesliding short 18, a stub-waveguide, coupled to modulator 16 provides ahighly reflective terminus for the radar energy and is tuneable forproviding frequency adjustment. A noise modulation source 21 is coupledbetween the modulator of point sources 12A and 128 for providing avariable noise level to the modulators. This noise level imposes amodulation onto the radar energy, resulting in a distorted, reradiatedsignal.

Similarly, a noise modulation source 22 is coupled between themodulators of point sources 12C and 12D for distorting energyre-radiated therefrom.

In operation, the glint simulator simulates the effects of scintillationon a radar system. It can be used as a laboratory tool or can beemployed in field testing of radar systems. Incident energy from atracking radar is intercepted by the glint simulator antennas 12. If nomodulation is applied to modulator 16, almost all of the interceptedenergy is re-radiated back to the radar, indicating a target or targetsof several points in space which are fixed with respect to each other.The amplitude of re-radiated energy is controlled by introducing amodulating voltage or a dc bias into modulator 16. The waveguide stub 18affords an adjustable short circuit for the radar frequency and reflectsthe radar energy back through the modulator, where it is coupled to andre-radiated by the antenna.

The point sources 12, repeaters, are separated by an arbitrary distanced, which may be determined by the target dimensions or previous signalsobtained from such target dimensions which indicate maximum pointsources. The target 10 cross-section is determined primarily by itsantenna physical aperture, and gain in the direction of the radar. Whena noise modulation that is out of phase with the received signal energyis applied to point sources 12, the apparent phase center of thesimulated target wanders. The individual targets, then, appear to varyin relative position. For a twocenter target only two point sources 12represent the target. A modulation signal 180 out of phase with theradar frequency causes the phase center of the simulated target towander from one side to the other, sometimes appearing as one target andagain as two for which the radar must compensate.

Modulator 16 can be a p-i-n (intrinsic-barrier diode) diode modulatorsimilar to the Hewlett Packard 5082-3000 series or other highlyefficient microwave semi-conductor diode circuits for modulating orswitching microwave signals. When active, the modulator causes thereflected signal to appear as a static reflection from the target.

Noise sources 21 and 22 provide random inputsv to modulator 16 allowingrandom amplitude and phase fluctuation in the reflected signal. Byproperly shaping the noise spectrum, desirable target characteristicsareintroduced. The particular noise source is selectable. Independent noisesources can be used with each point source, or a phase shift networkcoupled back to common reference signal can supply all of the pointsources, for example.

Utilizing the glint simulator, for testing a radar system, allows rapiddetermination and adjustment of the range over which a radar willcompensate for target induced scintillation errors. The degree ofscintillation compensation by installed radars may be readily verifiedand routinely checked.

Obviously many modifications and variations of the present invention arepossible in the light of the above teaching. For example, a travelingwave tube amplifier can function as the modulation element. However, forthis case, isolation is required between input and output to avoidunwanted parasitic oscillation, Thus, a single receiving antenna can beused to couple received energy through an isolator to the modulators forreradiation from respective modulator antennas. The basic principleremains the same modulation and spatial separation of reflected input tosimulate target and a sliding short connected to each of said modulatorsfor fine phase shifting of said radar energy.

2. A glint simulator as set forth in claim 1 wherein said noisemodulation means is a transformer having the secondary thereof coupledto said diode modulators.

3. A glint simulator as set forth in claim 2 wherein said plurality ofradar antennas are first and second point source antennas, and saidtransformer secondary is a single secondary connected in series withsaid diode modulators.

1. A glint simulator comprising: a plurality of radar antennas spaced apart for receiving and re-radiating radar energy as controlled point sources; controlling means coupled to each of said radar antennas for modulating said received energy prior to re-radiation thereof, said controlling means are p-i-n diode modulators for modulating said incident radar energy; noise modulation means common to said controlling means; and a sliding short connected to each of said modulators for fine phase shifting of said radar energy.
 2. A glint simulator as set forth in claim 1 wherein said noise modulation means is a transformer having the secondary thereof coupled to said diode modulators.
 3. A glint simulator as set forth in claim 2 wherein said plurality of radar antennas are first and second point source antennas, and said transformer secondary is a single secondary connected in series with said diode modulators. 